The present invention relates generally to an apparatus and process for producing potable water using a combination of reverse osmosis (RO) and dehumidification, and more particularly to a combination of shipboard RO and dehumidification to extract and store potable water from a saltwater supply.
A concomitant to an increase in global population is the need for potable water for human consumption, as well as for industrial, agricultural and other uses. Because the availability of freshwater supplies is limited by size, cleanliness and lack of accessibility, there exists a need for creating potable water from other sources. Stewardship measures such as conservation and reuse, while laudable, will not in and of themselves be sufficient to meet the increase in worldwide water demand.
The world's seas and oceans are the most notable source of yet relatively untapped water. Unfortunately, their high saline content precludes their use as a supply of potable water. Traditionally, the desalination of sea water is accomplished using land-based facilities, typically relying upon either active evaporative or reverse osmosis (RO) techniques. In the former, the salty water is first vaporized, then condensed in such a way as to isolate the relatively salt-free distillate. Active evaporation is expensive, requiring vast amounts of energy (typically in the form of a combustible heat supply).
The latter approach involves using high pressure to force the salty water through a membrane that is relatively impermeable to salt ions or other contaminants, thereby allowing a more pure form of the water to pass through the membrane. Traditional RO approaches typically involve some amount of pre-treatment, filtration, and final treating. Pre-treatment may include screen and physical filtration (often with carbon filters), as well as chemical pre-treatment, which may include scaling and biological prevention. From the pre-treatment, the water is then sent to the membrane for desalination and filtration. Membranes used in RO for the desalination of sea water come in four primary physical structures. These are spiral, tubular, plate and frame, and hollow fiber systems. Spiral systems are made up of two concentric tubes (typically about 8″ and 2″ in diameter) the length of which is dependent on system pressure and the concentration of solids in the raw water. The actual membrane is typically a flat sheet with one end open to the water and the other ending in the smaller of the two tubes. The membrane is then spiraled around the inner tube and placed inside the larger tube. Raw water enters the larger tube under pressure. Pure water then enters into the membrane and flows along the spiral until it is released into the inner tube where it is transported for final treatment. Concentrated brine then flows out the open end of the larger tube. Tubular and hollow fiber systems are essentially the same design differing only in their relative size. In both cases, membranes are cylindrical fibers or tubes placed in an outer tube. The outer tube is filled with pressurized raw water. The pure filtered water enters the tubes or fibers and is transported down these to final treatment. Concentrated brine flows out of the open end of the outer tube. Plate and frame systems involve a flat surface membrane with the filtering side exposed to raw water and the reverse exposed to the product water chamber. Pressurized raw water is exposed to the filter. The filtered pure water moves through into the collection chamber for processing. Final treatment involves the balancing and treating of mineral content in the water, as well as balancing the acidity of the water. Additionally, ultraviolet (UV) rays or chlorination can be employed to control future biological and pathogenic contamination.
As with the active evaporation process discussed above, land-based RO facilities suffer from various shortcomings. For example, because RO facilities generate significant quantities of dissolved solids and related effluent, release of such byproducts could be harmful if reintroduced in localized, concentrated form into the water supply from which it was derived. Such localized release of effluent would eventually cause the water being taken into the system to become concentrated enough that it can impact the performance of the RO system membranes. To ensure a relatively non-fouled RO water intake, the facility would need to be situated remotely from the point of effluent discharge. Similarly for evaporative systems, local nuisance concerns may mean that there are significant costs associated with keeping the facility at a suitable distance from population centers. In either situation, the solution tends to be cost-prohibitive.
One way to avoid the problems associated with land-based desalination (in particular, land-based RO desalination) is to use a shipboard RO system. In a conventional form, the high pressure requirements are satisfied by mechanical pumps. Such systems, while operationally suitable, are expensive, require significant amounts of energy consumption, and take up precious shipboard space. As an alternative, it has been reported that the necessary pressure differential can be achieved hydrostatically if the RO unit is submersed to a sufficient distance (for example, many hundreds of meters) beneath the ocean surface. Despite improvements in energy efficiency made possible by such a system, there remains a desire to increase the quality of potable water from ship-based platforms over that provided by these or related RO systems, as well as a desire to reduce the impact of RO-based desalination on the local environment from which the water was extracted.